The invention relates to a method of generating a clock, a clock generation device, and electronic apparatuses having such clock generation device.
Many electronic apparatuses employ microprocessors and other digital devices that require high-frequency clock signals. These fast digital devices are very sensitive to electromagnetic interference (EMI). A clock normally contains significant levels of EMI components at the fundamental and higher harmonic frequencies thereof.
Therefore, it is necessary that an electronic apparatus be designed to operate properly even under interferential electromagnetic waves. A standard of permissible EMI has been set forth for electronic apparatuses, so that electronic apparatuses be designed and manufactured to comply with the standard.
To do this, various structural improvements have been made to shield and/or minimize interferential electromagnetic radiation. These measures, however, require many extra parts and hence manufacturing steps, thereby rendering the manufactures costly.
Japanese Patent Application JP-A-5-90987 discloses a method of frequency-modulating a clock to generate a modulated clock having reduced levels of spectral components at the clock and higher harmonic frequencies. FIG. 1 is a block diagram representation of the method as mentioned above, showing its concept. FIGS. 2 and 3 show spectra of the clock and of the frequency-modulated clock.
In the example shown in FIG. 1, a clock generator 1 generates a clock CLK having a predetermined frequency fc. An FM signal generator 2 generates a square wave frequency modulation (FM) signal fm, which is fed to the clock generator 1 to frequency-modulate the clock signal CLK about the frequency fc.
Through such frequency modulation, the peak at frequency fc (dominant mode) of the clock will be dispersed into different components having reduced levels and spread about the frequency fc as shown in FIG. 3. The reduction in the level of the fundamental mode can be clearly seen by comparing the frequency-modulated spectrum with the non-modulated spectrum of the clock shown in FIG. 2. Thus, the EMI arising from the respective peaks will be reduced accordingly. The frequency fc can be the clock frequency or any other higher harmonic frequency of the clock.
In this conventional approach, EMI is reduced by the spectral dispersion of a peak at a given frequency fc through frequency modulation of the dominant peak. However, the degree of reduction of the peak depends on the range (or width) of the dispersion about the frequency fc. However, since the clock is a time basis of operation of the electronic apparatus, the frequency modulation is permissible only within a limited range where the dispersed modes will not disadvantageously affect the clock operation. Therefore, the range of frequency modulation cannot be made very large.
It is therefore an object of the invention to provide a method and a device for generating a clock which is frequency-modulated only in a limited range such that the modulated clock has a dispersed spectrum having sufficiently reduced levels at the fundamental and higher harmonic frequencies.
In accordance with one embodiment of the invention, there is provided a method of generating a clock, comprising steps of:
frequency-modulating a first signal having a first frequency by a second signal having a second frequency to generate a clock modulation signal; and
frequency modulating the clock by the clock modulation signal to thereby generate a doubly frequency-modulated clock in a predetermined range.
In accordance with another aspect of an embodiment of the invention, there is provided a clock generation device and an electronic apparatus utilizing such clock generation device, said clock device comprising:
a clock generation device having a clock generator for generating a clock having a predetermined frequency, said clock generation device adapted to receive a frequency modulation (FM) signal;
a modulation signal generator for generating a clock modulation signal by generating a first and a second FM signals having a first and a second frequencies, respectively, and by frequency-modulating said first FM signal by said second FM signal, wherein
said clock modulation signal is supplied to said clock generator to thereby frequency-modulate said clock to generate a doubly frequency-modulated clock having a range of frequencies.
In the present invention, the clock is doubly frequency-modulated in that the original system clock CLK is modulated by the clock modulation signal fm0 which is obtained from the first frequency modulation (FM) signal having frequency fm1 by frequency-modulating it by the second FM signal having frequency fm2. As a result, after dispersed in frequency by the first FM signal, the spectrum of the clock is again dispersed by the second FM signal. Consequently, peak levels of the components at the fundamental and higher harmonic frequencies of the clock are reduced greatly as compared with a conventional clock, thereby reducing its EMI to other electronic apparatus.
It should be appreciated that the range, fmax-fmin, of the frequency modulation of the clock used as the time-base of the system need not be large in order to reduce the EMI. In fact, it can be done in a rather narrow frequency range. This implies that the inventive clock generation will not disadvantageously affect the operations of other electronic apparatuses.
The clock modulation signal can be a signal having a successive triangular waveform (hereinafter referred to as triangular wave signal) or a signal having a sinusoidal waveform (hereinafter referred to as sinusoidal wave signal).
Further, the first signal can be a triangular/sinusoidal wave signal, while the second signal can be another triangular/sinusoidal having a lower frequency than the first signal.
This renders the spectral distribution of the frequency-modulated clock substantially flat, which causes much less EMI to other neighboring digital devices. The FM signal generators for generating the triangular/sinusoidal wave signals can be easily formed as two electronic devices on the same IC board as the clock generator, without any appreciable cost.
The clock generator can be a phase-locked loop (PLL) circuit that includes a variable frequency divider and a voltage-controlled oscillator (VCO). The clock modulation signal can be supplied to the variable frequency divider and the voltage controlled oscillator.
In these arrangements, the frequency-modulated clock modulation signal is supplied to the PLL circuit as a voltage control signal to control the frequency division, or to the VCO as a voltage control signal to control the input voltage or the voltage-controlled capacitance thereof. This permits smooth frequency-modulation of the clock.